Fischer-type alkylidyne-metal complexes

The chemistry of CR fragments ligating metal centers has been a topic of considerable interest since the discovery of the first alkylidyne-metal complexes in E.O. Fischer s Laboratory in 1973 (1). These ligands have been implicated in Fischer-Tropsch reactions (2) and in alkyne metathesis (5). Moreover, the isolation of stable compounds containing carbon-metal triple bonds completed the matrix of bond types represented here ... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Since this article is concerned with protonation reactions, some mention of earlier related work on the more classical alkylidyne-metal complexes is appropriate. The proton has proved a popular experimental probe for examining the reactivity of alkylidyne-metal complexes in studies attempting to establish the nature of the M=C bonding interaction. Calculations by Kosti6 and Fenske (20) on Fischer-type alkylidynes suggest that the metal-carbon bond is generally polarized —C ", but that the... 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

Complexes containing metal-carbon triple bonds in which the metal is in a relatively high formal oxidation state are frequently designated alkylidynes. Carbyne complexes are also sometimes classified as Fischer-type or Schrock-type, as in the case of carbene complexes. The distinction between these is discussed in Chapter 10. 

Alkylidenes have been prepared by reduction of alkylidynes, by C-H oxidative addition from alkyls, and by treatment of unsaturated metal clusters with diazoalkanes. In most instances, the alkylidene adopts a 112-rj coordination mode. However, alkylidenes with heteroatom substituents may also be found in terminal coordination modes. The latter are typically prepared by the Fischer-type carbene route (5ee Fischer-type Carbene Complexes (sequential addition of nucleophilic and electrophilic alkylating agents to carbonyl or isocyanide ligands), by condensation of metal fragments with mono- or dimetallic carbene complexes, or by C-H activation of alkylamines. These heteroatom substituted carbenes may also bind in a /u.3-jj mode, as in (12). 

Figure 1. a) Metal-ligand k orbitals of a Fischer-type alkylidyne carbonyl metal complex, b) Metal-ligand jt orbitals of the M(CR)(CO) fragment in the coupling plane. 

Alkylidene and alkylidyne complexes are also referred to as carbene and carbyne complexes. Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex 2.37, are called Fischer carbenes. In contrast, complex 2.38, wha e the metal ion is in a high oxidation state, is referred to as a Schrock carbene. 

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